教练在比赛中进行了一次换人。

The coach made a ___substitution in the game.

用新巴士逐步替代旧巴士成本很高。

The gradual ___substitution of old buses with new ones is costly.

在化学中，常见的取代反应涉及用一个原子替代另一个原子。

In chemistry, a common ___substitution reaction involves replacing one atom with another.